1. Field of the Invention
The present invention relates to a communication system for use in underground mines and more particularly to a mine communication system which includes a network of medium frequency transceivers and double-unit repeaters. The transceivers are magnetically coupled to electrical and natural waveguide conductors within the mine by loop antennas and are protected from incinerary conditions by an intrinsically safe current limiter circuit.
2. Description of the Prior Art
It has long been known that medium frequency (MF) electromagnetic waves propagate through natural media, such as coal and rock, as well as through electrical conductors such as track, wire rope and electrical wiring that exist in underground mines. Efforts have been made to exploit the propagation properties of MF signals to develop "wireless" communication systems. A wireless underground mine communication system would improve both mine productivity and mine safety. In a 1980 paper, Larry G. Stolarczyk proposed such a system. His cellular mine communication system exploited both the conductor mode of transmission and the natural waveguide mode of transmission for effecting MF radio communication within the mine. The system utilized a cellular repeater to provide a means for two mobile transceivers to communicate with each other. A communication link to the surface and to other repeaters was provided by a two-wire transmission line (i.e., a telephone line) over which voice signals in the audio frequency range were transmitted. L. Stolarczyk, The Design of a Cellular MF Radio Communication System for Underground Mining, Reprint from National Telecommunications Conference (Nov. 30-Dec. 4, 1980). Thus, this system suffered from the inability of surface stations and repeaters within the system to communicate with each other using radio frequency signals. This would become a serious problem if the telephone line was severed during a mine disaster, for example. This early system, which included a transceiver and a loop antenna attached to a vest worn by a miner, was described in more detail by L. Stolarczyk and R. Chufo in System Design and Performance of an MF Radio Communication System for Underground Mining, (Sep. 1981). The loop antennas used in these early systems were second order tuned loop antennas.
More recently, the MF wireless communication technique has been expanded to include a radio communication system which provides for the radio control of a mine train-loading operation. H. Dobroski and L. Stolarczyk, Control and Monitoring via Medium-Frequency Techniques and Existing Mine conductors, IEEE Transactions on Industry Applications, vol. 1A-21, No. 4 (Jul./Aug. 1985). In this system, MF radio signals are induced on existing conductors through the use of air core line couplers.
Many attempts have also been made at using ferrite couplers to induce MF radio signals on conductors. These suffer from the problem that no single ferrite material functions satisfactorily as both a receiving and a transmitting line coupler.
Finally, surveys have been published which review the attempts at developing various types of loop antennas for use in wireless mine communication systems. R. Lagace, D. Curtis, J. Foulkes and J. Rothery, Transmit Antennas for Portable VLF to MF Wireless Mine Communications, USMB Contract Final Report (H0346045), Task C, Task Order No. 1 (Arthur D. Little, Inc.) May 1977.
Loop antennas for use in mine communication systems that have been incorporated into a bandolier type garment have long been known. See, e.g. B. A. Austin and G. P. Lambert, An Interim Report on the Radio Communication System Installed Underground at Greenside Collery, Apex Mines Limited, Chamber of Mines of South Africa Research Report No. 39/77, Project No. CS1C10 (1977). The bandolier design suffers from the fact that as the miner's chest moves, the loop area of the antenna changes, thus changing the area and inductance of the antenna.
A separate direction in which mine communication methodology has developed is that of emergency communications. Two types of emergency mine communication systems are the seismic method and the borehole method. In the seismic method, a trapped miner transmits seismic vibrations by pounding on a rail or roof bolt. These signals are detected by surface geophones. After computer analysis of the arrival times of the seismic signals, the location of the trapped miner can be determined. The seismic method has proven inadequate because the deployment of geophone arrays is time consuming and voice communication is impossible. Additionally, the technique requires that the miner not be seriously injured so that he can pound on a rail or roof bolt in order to be detected.
In the borehole method, probes are lowered down boreholes in order to provide two-way voice communications with trapped miners. This method is not satisfactory because set-up drilling is time consuming and useless if the exact location of a trapped miner is not known.
A safety requirement for all electrical equipment used in mines is that the equipment be intrinsically safe. Intrinsically safe equipment is incapable of releasing sufficient electrical or thermal energy, under normal or abnormal conditions, to cause ignition of a specific hazardous atmosphere mixture in its most easily ignited concentration. IEEE Standard Dictionary of Electrical and Electronics Terms, 3rd Edition, p. 463 (1984). To satisfy this requirement, the Mine Safety and Health Administration (MSHA) and the U.K. Health and Safety Executive (HSE) require that batteries be protected with a fuse and series resistor circuit. The fuse is designed to blow out before the temperature of the resistor reaches a certain temperature. A disadvantage of this circuit is that it requires that larger batteries be used to compensate for the voltage drop across the resistor. The use of larger batteries increases the size of mine equipment and decreases the capacity of the batteries.